1. Field of the Invention
The invention relates in general to an apparatus for reducing energy loss, and more particularly to an apparatus for saving energy in a liquid crystal device (LCD).
2. Description of the Related Art
The progress of display technology brings more innovative display devices for users. Because LCDs are low radiation, low power, and compact, they are gradually substituted for higher radiation, larger, conventional cathode ray tube (CRT) displays in the high-end market. Nowadays, notebook computers and projectors are equipped with LCDs. Besides, more and more desktop computers' users select LCD monitors to substitute for conventional CRT displays.
The display panel of an LCD is formed with a front plate, a rear plate, and the cavity between the front and the rear plates, wherein the cavity between the front and the rear plates is filled with liquid crystal molecules. In a typical transmissive LCD, its display panel is equipped with a back lighting source. The fraction of light transmitting through the display panel is called light transmissivity. The light transmissivity determines the brightness of the display panel. In addition, how the liquid crystal molecules in the cavity between the front and the rear plates are arranged determines the light transmissivity of the display panel. Further, the arrangement of these liquid crystal molecules depends on the voltage across the front and the rear plates. Thus, the brightness of the display panel can be controlled by applying different voltage across the front and the rear plates.
It should be noted that the light transmissivity of the liquid crystal molecules is only related to the value of voltage across the front and the rear plates, and is not related to the polarity of the voltage applied to the front and the rear plates. For example, if a pixel is supplied with two voltages separately in the same value but opposite in polarity, the pixel will have the same light transmissivity correspondingly. In particular, if voltages in the same polarity are continually applied to the pixels of the LCD, the liquid crystal molecules of the pixels may deteriorate. Since the light transmissivity of pixels is independent of the polarities of voltages applied to the pixels, the liquid crystal molecules can be prevented from deteriorating by alternately changing the polarity of the voltages applied to them. Such approaches are called polarity inversion.
In terms of polarity inversion, driving methods for typical LCD display panels can be categorized into three methods as follows: frame inversion, column inversion, and dot inversion. The following is their brief descriptions.
FIGS. 1A-1F respectively show a portion of the pixels of a display panel driven by different polarity inversion driving methods. The portion of pixels is indicated by squares, and plus (+) or minus (−) signs in the squares indicate that the associated pixels are supplied with positive voltages or negative voltages, individually. FIGS. 1A and 1B illustrate the frame inversion driving method for driving a display panel. Alternately, if all pixels of the display panel are fed with positive voltages at a time instant, as shown in FIG. 1A, negative voltages are fed into the pixels at the next time instant, as shown in FIG. 1B. This driving method is to alternately change polarity of all the applied voltages to the pixels of the whole display panel and is thus referred to as the frame inversion driving method.
FIGS. 1C and 1D illustrate the column inversion driving method for driving the display panel. At a time instant, some column of pixels, such as even pixel columns, are supplied with positive voltages while the other pixel columns, such as odd pixel columns, are supplied with negative voltages, as shown in FIG. 1C. At the next time instant, the even pixel columns are supplied with negative voltages while the odd pixel columns are supplied with positive voltages, as shown in FIG. 1D. Since the polarities of voltages applied to the pixels of the display panel are changed on the basis of whole lines (e.g., columns), this driving method is called the column inversion driving method.
FIGS. 1E and 1F illustrate the dot inversion driving method for driving the display panel. In this driving method, every pixel can be viewed as a dot, and every dot is surrounded by other dots with voltages in inverse polarity. That is, if a pixel is supplied with a negative voltage, the adjacent pixels are supplied with respective positive voltages. At the next time instant, the polarities of each of the pixels will be changed.
The polarity inversion driving methods described above can avoid the liquid crystal molecules from deteriorating and can improve the display quality of the LCD panel. However, a large amount of energy loss would occur in the driving circuit of the LCD panel when voltages applied across the front and the rear plates drop and rise between inverse polarities.
FIG.2 shows a portion of the equivalent circuitry of an LCD panel driven by a driving circuit using column inversion or dot inversion. Suppose that when a pixel 202 and a pixel 204 have identical brightness, the pixel voltage VC1 across the capacitor C1 of the pixel 202 and the pixel voltage VC2 across the capacitor C2 of the pixel 204 are equal in value and opposite in polarity. Referring to FIG. 2, a conventional method of reducing energy loss in polarity inversion uses transmission gates coupled between two adjacent data lines, such as a transmission gate 206 coupled between adjacent data lines D1 and D2. At time t1, the driving circuit enables a scan line S and turns on the transmission gate 206. In this way, the pixel capacitors C1 and C2 are electronically coupled together; one of them will discharge and the other will be charged so that the pixel voltages VC1 and VC2 of the two pixel capacitors C1 and C2 approach to (VC1+VC2)/2. This process is referred to as charge sharing. At time t2, voltages in polarities opposite to that at the time t2 are fed into the data lines D1 and D2 respectively to change the pixel voltages VC1 and VC2 into pixel voltages VC1′ and VC2′ opposite to the pixel voltages VC1 and VC2 in polarity, respectively. In other words, before the polarity inversion for the pixels, the pixel voltages of the pixel capacitors C1 and C2 have been made nearly equal to the average of the pixel voltages at the time t1 so that, during polarity inversion, the changes in the pixel voltages of the pixel capacitors C1 and C2 are made smaller than the conventional ones. Thus, the energy loss is reduced during polarity inversion.
The conventional method above results in reduced energy loss. However, the saved energy is small as compared to the total energy loss and an amount of energy is still dissipated in the form of heat. The operating temperature of the LCD is then increased with time. If the operating temperature is higher than a maximum operating temperature, the performance of the internal circuits would be degraded and the lifetime of the LCD would be shortened.